Technical Field
Embodiments of the invention relate generally to power electronics, e.g., switching devices. Particular embodiments relate to power electronic devices in which plural switching devices are packaged together.
Discussion of Art
Trains typically feature a number of cars that are pushed or pulled by a locomotive. The locomotive has traction wheels engaged with the track. In modern designs, electric wheel motors drive the traction wheels. The electric wheel motors are powered via electrical distribution from one or more engine-driven generators housed within the locomotive. The traction wheels and wheel motors can be reversibly configured, to also act as brakes for slowing the locomotive.
Similarly, in the mining industry, large off-highway vehicles (“OHVs”) usually employ electrically motorized wheels for propelling or retarding the vehicle. In particular, OHVs typically include a large horsepower diesel engine in conjunction with an alternator, a main traction inverter, and a pair of wheel drive assemblies housed within the rear tires of the vehicle. The diesel engine is directly associated with the alternator such that the diesel engine drives the alternator. The alternator powers the main traction inverter, in which semiconductor power switches commutate the alternator output current to provide electrical power to electric drive motors of the two wheel drive assemblies.
In both locomotive and OHV applications, solid-state power converters are used to provide high voltage current from the generators or alternators to the wheel motors. Such power converters include inductive coils to step down the voltage as well as semiconductor power switches to commutate the current. Although the above-described applications are typical, it will be appreciated that power converters can be used in many other settings.
Generally, operation of a power converter is accomplished by applying alternately two different gate voltage levels to individual semiconductor power switches via corresponding gate drive units. Exemplary semiconductor power switches include IGBTs as well as BIGTs. Semiconductor power switches, however, are subject to cyclic thermal stresses. More specifically, while driven by a gate voltage, each power switch conducts significant current in a forward direction at a relatively small voltage drop across the switch. Despite the relatively low voltage across the forward-biased power switch, resistive heating nonetheless occurs. As will be appreciated, when gate voltage is removed, each semiconductor ceases to conduct (except for leakage current). Thus, with proper thermal design, a power switch not driven by gate voltage should cool toward ambient temperature.
Although durability is a consideration in semiconductor design, electrical design constraints entail that the various layers of the semiconductor power switches are fabricated from materials having differing thermal properties; in particular, differing coefficients of thermal expansion. As a result, over time, thermal stress can potentially cause delamination, debonding of terminals, or fatigue cracking. Thermal stress can also cause electrochemical failures such as current filamenting and Kirkendall void formation.
Thermal stress effects can be rendered more predictable, and can be mitigated, by maintaining the heating/cooling cycle within a design envelope defined to minimize temperature swings despite continual on/off cycling. For example, semiconductor power switches may be specially packaged with reference to a heat sink or coolant conduit, such that all of the power switches mounted on the heat sink are at a generally common temperature. Such switches are commonly referred to as “isothermal switches.”
Structural configurations of isothermal switches, however, may present challenges in electrical circuit balancing. For example, switch electrical connection points are split into multiple terminals to allow high currents. Terminal connectors are then used to provide a common electrical connection of the multiple terminals. Typically, a terminal connector includes a conductive layer sandwiched between insulative layers. The terminal connector is connected to the multiple terminals most often by screws, although other fastening methods may be used. While such packaging arrangements often provide operational benefits with isothermal switches, they may present challenges in keeping the circuit “electrically balanced” such that all electrical terminals belonging to the same electric point see the same current flow and hence the same power losses. Electrically unbalanced systems (those where electrical terminals belonging to the same electric point do not see the same current flow and power losses) may show undesirable electric behaviors in the circuit, since the switching devices will be at different operating points far from the ones foreseen by design and not balanced.
In view of the above, it may be desirable to optimize both electrical and thermal parameters of semiconductor switches in a power converter in order to control resistance and keep stray inductance values as low as possible.